Build view with data rows from one of two views.

Question

Hello. I've got problem with PL/SQL.
I have built two PL/SQL views. Now I have to write a third
view based on the data from "view_1" or "view_2". "view_2" contains all data rows
but "view_1" contains special information. If the needed data rows (it's
always a tupel of two rows) are available in "view_1" i have to use these data.
Are they not available in "view_1" i have to get it from the second view.
I've tried it with the DECODE function and a procedure I've written but I think it's
the completly wrong way because I have to search for a tour_id and there are always
two shipments for one tour.
It looks something like:
>
SELECT
DECODE(FNC_OPERATION_INSTRUCTION_EXISTS(T1.TOURNO),0,
(SELECT SHIPPING_NO
FROM VIEW_SHIPPINGADRESS
WHERE TOUR_ID=TO_CHAR(T1.TOURNO)),
(SELECT SHIPPING_NO
FROM VIEW_SHIPPINGADRESS_OPERATION_INSTRUCTION
WHERE TOUR_ID=TO_CHAR(T1.TOURNO)) "SHIPPING_NO"
FROM
TB_TOURS     T1
WHERE
But for sure I've got the error "ORA-01427: single-row subquery returns more than one row."
because there are always two rows for one T1.TOURNO.
I think that it's a standard problem but I also think that I'm on the wrong way.
I will be very thankful for any hint.
Thanx,
Christoph

Answer 1

select col1,
       col2,
       col3
  from  (select (select decode(count(*),0,0,1) cnt
                   from view_1
                  where ...)) chk,
                 col1,
                 col2,
                 col3
           from view_1
          where ...) tab1
where tab1.chk = 1
union all
select col1,
       col2,
       col3
  from  (select (select decode(count(*),0,0,1) cnt
                   from view_1
                  where ...)) chk,
                 col1,
                 col2,
                 col3
           from view_2
          where ...) tab1
where tab1.chk = 0note: untested.